Pressure vessels for use at low temperatures



March 9, 1937. J. c. DUCOMMUN ET AL 2,073,053

- PRESSURE VESSELS FOR USE AT LOW TEMPERATURES Filed Dec. 22, 1934 is E 8 farms/zed Charpy l/a/Lua-F.Lbs. I

Inventors:- Jesse GDacomma/z- Joseph 14/. Ycznt ATTORNEY Empercature ..F

Patented Mar. 9, 1937 UNITED STATES PRESSURE VESSELS FOR USE AT Low TEMPERATURES Jesse C. Ducommun, Hammond, and Joseph W. I Yant, East Chicago, Ind., assignors to, Standard Oil Company, Chicago, 111., a corporation of Indiana Application December 22, 1934, Serial No. 758,771

2 Claims. (Cl. 148-31) This invention relates to improvements in pressure vessels for use atlow temperatures and methods for making such pressure vessels, and more particularly to pressure vessels made from 5 steel of relatively low carbon content intended for use in temperature ranges extending below Ordinary low carbon steel undergoes a relatively rapid increase in embrittlement at temperatures below freezing and at reduced temperatureis therefore subject to cracking andbreakage by'impact. This undesirable behavior has become exceedingly troublesome in the construction of pressure vessels for low temperature dewaxing of lubricating oils, by the newer processes employing liquefied hydrocarbon gases such as propane, butane and ethane, involving temperatures from 30 to -l00 F.

Itis, therefore, an object of the invention to provide such pressure vessels made from comparatively low carbon steel which possesses a relatively high degree of toughness at temperatures as low as -100 F.

Other objects, advantages, and uses of the invention will become apparent after reading the following-'specification and claims.

Our invention is applicable to carbon steel having a range in carbon content from .05% to 25% but may be applied, if desired, to steel having as high as carbon content.

In the drawing we have shown a chart indicating Charpy values for low carbon steel before and after treatment according to the invention. In treating steel having a carbon content within the aforementioned range for service at low temperatures, the metal, as commercially received, is first placed in a furnace and heated uniformly throughout to a temperature above its upper critical point, thereby to cause the steel and carbon to form a homogeneous solid solution, at which time it may be said to be in the gamma iron state.

'After the entire mass has arrived at the desired furnace temperature, above the upper critical point, the steel'is then quenched in water. thereby. creating a homogeneous fine grained structure. is heated in the furnace is preferably higher than the temperature desired for quenching since there will be some cooling of the steel during the transfer thereof from the furnace to the quenching vat. The steel therefore cools down toward its upper critical, or A r3 point, which is slightly lower than the upper critical or A03 point ob- 65 tained on heating.

The temperature to which the steel If desired, other liquids than water may be used for quenching the steel. We prefer to employ liquids having high heat conductivities so as to provide for rapid cooling of the steel. There 'are many such liquids to choose from, including sulfuric acid, strong brine solutions, and oils. Sulfuric acid, although it does not have as high a specific heat capacity as water, possesses advantages over water, for this purpose, because it has a boiling point considerably higher than water and the formation of a vapor envelope about the steel minimized. j

After quenching, the steel may be found to possess internal strains as a result of the sudden lowering of temperature during quenching,

and these may be especially'serious when present in pieces of large dimensions.

Such internal strains may be removed and the steel made ready foruse by placing itin a furnace and soaking at a temperature near. to the lower thermal critical point and preferably within 75 F. thereof and in holding the steel at such temperature sufficiently 'long to. spheroidize the cementite. The time during which the steel is thus soaked may be from one to twenty hours, determined largely by the cross-sectional dimensions of the mass treated. For average work,

three hours have been found sufficient. If desired. however, the time during which the steel is maintained at soaking temperature may be varied to control, to some extent, the hardness of the finished product, although under all cir- -cumstances sufficient time should be allowed to phase of the process:

.05% carbon 166l F. .12% carbon 1641 F. .16% carbon 1602 F. 24% carbon 1634" F.

The following is a table representative of the lower critical temperatures with respect to carbon content upon which draw-back or soaking temperatures are determined:

.05% carbon 1275 F. .12% ca bon 1279 F. 16% carbon 1287 F. 24% carbon 1285 F.

From the above tabulations, it will be apparent that the quenching temperatures may lie within the range of 1600 to 1800 F. for low carbon steel and that the temperature at which the steel is soaked for the purpose of relieving quenching strain may lie within the range of 1275 to 1400" F., although we prefer to narrow the latter range to between 1275 and 1325 F. We have found as stated, that in commercial practice it is most desirable to heat the steel in a furnace running hotter than the quenching temperature in order that the steel at the time of immersion be above the upper critical temperature.

Low carbon steel prepared as above described may be used advantageously in almost any form of structure exposed to atmospheric temperatures during the winter months in temperate and northern climates, wherein shock and impact to the structure is likely to occur. Thus the steel is particularly applicable for use in refinery apparatus which must necessarily be subjected to a wide range of temperatures and to some mechanical abuse as during riveting, drilling, cutting and similar repairs made when cold, and wherein the structure is erected out of doors and subjected to exposure at all times to climatic changes in temperature. The shock resistance of the steel is also valuable in apparatus for conducting various industrial processes wherein the material in process of preparation undergoes treatment at temperatures below freezing. This is notably the case in handling liquefied gases under high pressure, where the steel is subjected to extremely rapid cooling by evaporation, and a crack or weakness in the steel resulting from impact failure at low temperatures may cause disastrous failure at ordinary temperatures and correspondingly high pressures. Such operations are described in a paper by Bahlke, Giles and Adams in the Proceedings of the American Petroleum Institute at the Mid-Year Meeting, May 17, 1933.

With reference to the drawing, we have indicated therein a chart containing curves representing the Charpy impact values of a lap welded pipe and a seamless steel tubing prior to and subsequent to treatment, according to the herein described process. The Charpy test specimens from the pipe and tubing tested were .394" wide, .250" thick, 2.165" long and notched with a milling cutter .0785" wide with a cutting edge rounded to a radius of .03925". A metal thickness or .197 remained to be broken after the notch was milled. The curve I--a represents the Charpy impact values of the pipe as commercially received indicating that. at 25 F. the pipe had a Charpy value of less than 2 ft. lbs. and that after treatment, as described, and as indicated by the curve Ib at the same temperature the Charpy impact value had risen to a point above 55 ft. lbs. The

Mn P S Si Ni I. Lap welded pipe .12 40 042 042 008 05 II. Seamless steel tubing"... .14 .49 .009 .027

While we have described our invention by showing its application to specific examples, these are given for the purpose of illustration only and are not intended to limit the scope of the invention, the breadth of which is to be measured only by the limitations of the following claims.

We claim:

1. A pressure vessel made from a low carbon steel containing from about .05% to about 25% carbon, having a Charpy impact value in excess of 50 ft. lbs. at 25 F. and possessing a.microcrystalline structure which renders said pressure vessel suitable for use in the handling of liquefied gases wherein said pressure vessel is cooled rapidly by evaporation of said liquefied gases to temperature below 0 F., said micro-crystalline structure being engendered by a heat treatment comprising quenching said steel pressure vessel from a temperature within the range from about 1600 F. to about 1800 F. and subsequently soaking the quenched pressure vessel at a temperature within the range from about 1275 F. to about 1400 F. for a period of at least one hour and thereafter gradually cooling to ordinary temperatures.

2. A pressure vessel made from a low carbon steel containing from about .05% to about 25% carbon, having a Charpy impact value in excess of 50 ft. lbs. at 25 F. and possessing a microcrystalline structure which renders said pressure vessel suitable for use in the handling of liquefied gases wherein said pressure vessel is cooled rapidly by evaporation of said liquefied gases to temperatures below 0 F., said microa-crystalline structure being engendered by aheat treatment comprising quenching said steel pressure vessel from a temperature within the range from about 1600 F. to about 1800" F. and subsequently soaking the quenched pressure vessel at a temperature within the range from about 1275 F. to about 1325 F. for a period of at least one hour and thereafter gradually cooling to ordinary temperatures. 

